Microalgae can produce a variety of chemical products by fixing carbon dioxide through photosynthesis. Some microalgae can produce aliphatic hydrocarbons, for example, the hydrocarbon production of botryococcus can reach 15%-75% of dry cell weight; some microalgae can accumulate glycogen; some microalgae can accumulate glycerol, wherein the lipid content of many microalgae can reach 60% or above of the dry cell weight. The average combustion heat of fuel oil obtained by pyrolysis of algae biomass can be up to 33 MJ/kg. Microalgae can be cultured in seawater, salty water or semi-salty water, avoiding scrambling for land and freshwater resources with crops, and can be cultured with waste water. So, microalgae could be an important source to obtain biological resources in regions lack of freshwater or with barren land. Therefore microalgae are expected to be important sources for future energy and chemical products.
The carbon in microalgae cells accounts for more than a half of the dry cell weight of the cells, and the algae cells can fix carbon dioxide as their own components through photosynthesis in growing process, so that the supply of carbon sources should be assured in the culture medium in the algae culture process. The inorganic carbon sources in the algae culture medium exist in three forms, namely HCO3−, CO32− and free CO2. The ratio of the three forms of carbon sources in water varies with the pH value of the culture medium. If NaHCO3 is used as the carbon source, with the dissociation of HCO3− and the utilization of CO2, the pH value of the culture medium rises continually, more than half of the added NaHCO3 is converted to Na2CO3 which could not be utilized by algae, resulting a waste and considerable consumption of carbon source; moreover, the medium is difficult to be recycled because of the rise of its pH value. If CO2 is used as the carbon source, which is directly utilized by the microalgae, then the problem that the pH value of the culture medium rises can be avoided, which is beneficial for maintaining an desirable culture environment and allows the medium to be used repeatedly or for an extended period.
Cultivation in an open pond is a traditional and simple mode of microalgae culture, and is also recognized as a mature microalgae culture technology currently. It has the advantages of having a simple construction and being easy to operate, and has been applied to commercial production of spirulina, chlorella and Dunaliella salina (Chaumont D., J. Appl. Phycol., 1993, 5:593-604; Richmond A., Progress in Physiological Research, Vol. 7, Biopress, Bristol., 1990, 269-330; Borowitzka Bioresource Technology, 1991, 38: 251-252). However, the depth of the culture solution in a traditional open pond is usually kept at 20-30 cm, if the CO2 is directly aerated into the open pond in a bubbling way, due to the very short residence time of the bubbles in the culture solution, the absorption efficiency of the CO2 is very low—only 13%-20% of the CO2 is absorbed (Becker E W, Microalgae: biotechnology and microbiology, Cambridge University Press, Cambridge, 1994, pp 293).
Ferreira et al. (Ferreira B S, Fernandes H L, Reis A and Mateus M. Microporous hollow fibers for carbon dioxide absorption: mass transfer model fitting and the supplying of carbon dioxide to microalgae cultures. Journal of Chemical Technology and Biotechnology, 1998, 71: 61-70) utilized a hollow fiber membrane to enhance gas-liquid mass transfer, so as to improve the absorption efficiency of the CO2, but the method is high in cost, and the hollow fiber membrane is prone to be fouled.
As for the method of L I Yeguang, H U Hongjun, ZHANG Liangjun and CHEN Zhixiang (Study on CO2 supply technique for spirulina production, Journal of Wuhan Botanical Research, 1996, 14 (4): 349-356), a gas-cover in size of several square meters is arranged on the surface of microalgae culture solution, and carbon dioxide gas is introduced into the gas-cover, so that the carbon dioxide is transferred into the culture solution through the water surface shrouded by the cover. The problems with this method are as follows: the specific interfacial area for gas-liquid exchange is small; the mass transfer rate is lowered down due to accumulation of oxygen and nitrogen in the gas-cover, and the oxygen and nitrogen need to be ventilated frequently, so part of the carbon dioxide in the gas-cover is wasted; for a gas source containing low-content carbon dioxide, the absorption efficiency of the carbon dioxide is very low; when the pressure in the gas-cover is slightly higher, the gas may leak out from the edge of the gas-cover through the liquid surface outside the gas-cover. Groove type carbon supplement method (CN200610018771.9, Device for Supplementing Carbon Dioxide into Microalgae Cultivation Pond) is as follows: a deep groove is dug and located beside a culture pond to enable the culture solution to flow through the deep groove, a gas sparger is arranged at the bottom of the groove, through which the carbon dioxide is supplied into the culture solution. The method may disrupt the spatial layout of the traditional open pond; and furthermore, the culture solution in the groove is not mixed well, the bottom of the groove becomes a dead zone for mass transfer after being saturated with carbon dioxide after a period of sparging, and then the deep groove does not function as a mass exchanger.
CONG Wei et al. (CN200510126465.2, Carbon Supply Device for Large-scale Culture of Microalgae and its Application and Use) develop a trap type carbon supplement device for directly supplementing CO2 into the culture solution in an open pond, wherein the culture solution can form a circulation in the trap type carbon supplement device, thereby the time for gas-liquid contact is greatly prolonged; moreover, gas is supplied from the bottom of the trap type carbon supplement device; so that the absorption efficiency of CO2 is greatly improved. However, the carbon supplement device increases the flow resistance in the open pond, thereby resulting in increased electrical energy consumption for driving the fluid with a paddle wheel under the same flow velocity, and relatively more construction workload.